The present invention relates to a method, by which a combination of four colors is determined, and an apparatus for the method, in a four color printer in which a target color is color-reproduced when four colors of Y (Yellow), M (magenta), C (Cyan), and K (Black) are combined, and specifically, to the method and apparatus in which reproducibility is improved when an amount of K is specifically determined.
When full-color printing is conducted by usual methods of printing, thermal transfer, ink jet, or electrophotography, four colors of Y, M, C, and K are generally used as output colors in color printing.
Conventionally, various types of methods, by which the color is expressed colorimetrically in four colors, have been tried.
For example as the first method, the following method has been disclosed: a new K-amount is determined when the K-amount of a color solid, which is formed by conditions of Y=0, M=0, and C=0, is weighed with a predetermined amount; and new Y, M, C amounts are colorimetrically determined according to the new K-amount (Theses of the 9th Chromatic Engineering Conference, Flexible UCR using L * a * b * the first report, A method of considering new UCR which can be used for both letter and half tone reproduction, written by Shinji Kita and Akira Kokatsu in 1992).
As the second method, the following method has been disclosed in which a combination of Y, M, C, K, by which a target color is reproduced from four color solids determined under the condition of Y=0, M=0, C=0, K=max., is found (refer to Japanese Patent Publication Open to Public Inspection No. 136848/1990). Due to the foregoing, the total amount of ink can be reduced when a maximum K-amount is used while the maximum color region, which can be reproduced in the four color printer, is kept constant.
As the third method, the following method in which a combination of Y, M, C, K is found, by which a target color is reproduced from the four solids determined under the conditions of Y=max., M=max., C=max., K=0, has been disclosed in Japanese Patent Application No. 266718/1992 (published as Japanese Patent Publication Open to Public Inspection No. 6-98161). In this method, the minimum K-amount is used and the total amount of ink is increased. However, the gradation properties are improved because noises are cancelled when the number of colors to be used is increased.
However, in these conventional methods, a portion is generated in which the K-amount is suddenly changed when the target color is changed. Accordingly, the conventional methods have the following common problem in which a pseudo profile is generated when the printing quality fluctuates.
Theoretically, the same target color can be reproduced by combinations of Y, M, C, K even when the K-amount is changed. Practically, because the printing quality fluctuates, errors are caused by the combination in which the K-amount is largely changed. Therefore, even when increasing and decreasing directions of a predetermined color element, for example, brightness, in the target color, (the directions of the change correspond to those of density of a reproduced color), are matched with those of the K-amount, the K-amount is not smoothly changed and the pseudo profile is easily generated, in the boundary portion between a portion in which the K-amount rises or smoothly changes from a region, in which K is not used, to the region, in which K is used, and a portion in which the K-amount is fixed at the maximum.
Further, image signals (R, G, B) of an image, which is displayed in a target device such as a CRT, are converted into image signals (Y, M, C, K) which are reproduced in a printer, which is used as a device for reproduction. In this case, when a function transformation by an LUT (Look Up Table) and an interpolation by an interpolation device are combined, the interpolation is conducted according to data of the color solids located on both sides between which the non-smooth portion is sandwiched. In this case, there occurs the following problem in which interpolation errors become large because one data portion is connected with the other data portion in the shape of a polygonal line, which is not smoothly connected (Japanese Patent Disclosure 53-123201 (1978)).
Further, in the first method, because a K-amount to reproduce the target color is calculated in combinations which are made under the conditions of Y=0, M=0, C=0, there is a region which can not be substantially used in the largest number of color regions reproducible in a four color printer. That is, when the K-amount is maximum, although two colors can be used at the maximum in Y, M, C, three colors at maximum can not be used. Simply, when a printer, in which Y, M, K can be used, is supposed, and the color region is shown on a two-dimensional coordinate, the color region corresponds to an oblique line portion shown in FIG. 5 (P. Hung, "Colorimetric Calibration for Four-color Printers' Journal of the Society of Photographic Science and Technology of Japan 56(2), 112-122 (1993)).
Further, when a K-amount to reproduce a target color, that is, a ratio of a K-amount to the maximum K-amount obtained under the conditions of Y=0, M=0, C=0, is arbitrarily determined, there occurs the following case in which the K-amount is decreased and replaced with other three colors, in the case of a color located near the boundary of the reproducible color region. For example, in the case where the K-amount is 90 (the maximum amount is 100), M, C.gtoreq.60 under the condition of Y=0, it is necessary that amounts of Y, M, C are increased by a decreased amount of K when the K-amount is decreased to 45. Accordingly, amounts of M and C are increased to the amounts larger than 100, and go out of the color region, so that the color can not be correctly reproduced.
On the other hand, as described above, in the third method, the minimum K-amount is used so as to increase the gradation properties. However, practically, there is a case in which it is preferable to increase the gradation properties with the chroma or tone, and a case in which it is preferable to decrease the gradation properties. In this connection, in the second and third methods, there is no degree of freedom in which the ratio of the K-amount is adjusted corresponding to the color so that the desired image quality can be obtained, because these two methods are restricted to the use of the maximum K-amount, or to the use of the minimum K-amount.
The present invention has been considered according to these conventional problems. The first object of the present invention is to provide a method in which a K-amount is appropriately set so that a pseudo profile can not be generated, when combinations of Y, M, C, K are determined in a four color printer in which Y, M, C, K are used. In addition to the above, the first object of the present invention is to provide a method in which the accuracy of an interpolation can be increased, and a color can be adjusted to include a preferable ratio of a K-amount corresponding to chroma or tone.
The second object of the present invention is to provide a method in which: the reproducible color region is kept constant by Y, M, C, K; the pseudo profile is not generated; and the accuracy of interpolation can be increased.